Disposable diapers, sanitary napkins and other sanitary materials have an absorber body for absorbing bodily fluids and other liquids, a soft, liquid-permeable top sheet on the side that contacts the body, and a liquid-impermeable back sheet on the side away from the body. The absorber is normally made from a mixture of pulp or another fibrous substance and a water-absorbent resin.
There has been increasing demand in recent years for thinner, more lightweight sanitary materials to resolve problems of design, distribution, garbage disposal and the like. The most common method currently used to meet these demands in sanitary materials is to reduce the amount of fiber or other support material for the water-absorbent resin in the sanitary material and use larger amounts of water-absorbent resin. Such sanitary materials having a smaller proportion of hydrophilic fiber and a higher proportion of water-absorbent resin may be better at simply storing liquid, but are not necessarily good at distributing and dispersing the liquid when the diaper is actually used. That is, the large amount of water-absorbent resin turns into a soft gel when it absorbs liquid, resulting in a phenomenon called gel blocking which greatly inhibits dispersal of the liquid. Once gel blocking occurs the resin no longer performs properly, and not only does it absorb less water but absorption speed decreases.
The proportions of hydrophilic fiber and water-absorbent resin must both be restricted in order to avoid this problem and maintain the absorbent properties of the absorber, so there are limits on reducing the hydrophilic fiber and reducing the thickness of the sanitary material. Moreover, pulp is most commonly used as the fiber in sanitary materials, especially virgin pulp is normally used because of its cleanliness, therefore the use of large quantities of fiber increase of burdens on forest resources.
Methods that have been proposed for preventing the gel blocking that occurs when amount of fiber is reduced and large amounts of water-absorbent resin are used include a method using two different water-absorbent resins with different absorption abilities (see for example Patent Document 1), a method using a composition containing a cationic ion-exchange hydrogel-forming polymer and an anionic ion-exchange hydrogel-forming polymer (see for example Patent Document 2) a method using a water-absorbent resin crosslink density on the surface of which is high (see for example Patent Document 3), a method using a water-absorbent resin the salt density of which near the surface is lower than the overall salt density (see for example Patent Document 32), and a method extruding and foaming a mixture of a water-absorbent resin and a thermoplastic resin into a sheet (see for example Patent Document 4). However, these suffer from such problems as high cost and inadequate absorption properties for use in an absorber with a high water-absorbent resin concentration. Moreover, because these methods reduce the relative amount of hydrophilic fiber, which serves to hold the water-absorbent resin, the particles of absorbent resin tend to become unevenly distributed before use or to move around during use. The absorbent product loses its shape when the water-absorbent resin shifts from its intended position in this way, and the excreted urine or other liquid does not contact the water-absorbent resin in the absorbent product, resulting in leakage.
In another method, the method of mixing the fiber and water-absorbent resin is fine-tuned so as to prevent uneven distribution and blocking of one part of the water-absorbent resin by another part (see for example Patent Document 5). In this method, the degree of mixing is high because the water-absorbent resin and pulp are first mixed in water and then dry mixed with hydrophilic fiber and formed into a web with air. However, the problem is that the water-absorbent resin may clog the web during the process of air formation. Once this occurs, it becomes necessary to remove it from the drum, decreases productivity. It is also preferable to use a relatively hard absorbent resin in order to prevent clogging and facilitate removing when clogging occurs. In general, hard water-absorbent resins tend to have low absorption capacity, making it necessary to use a large quantity of water-absorbent resin in order to absorb the target amount of liquid. This method is also unsatisfactory in terms of preventing movement and uneven distribution of the water-absorbent resin in the product.
In order to resolve these problems, particularly the problem of movement and uneven distribution of the water-absorbent resin, methods of bonding the water-absorbent resin onto a support have been studied. Examples include a method of embossing an absorber, a method in which a thermoplastic binder fiber is included in an absorber comprising a water-absorbent resin and hydrophilic fiber, and the absorbent body is then thermally fused, a method in which a synthetic resin with high recovery from deformation is included in an absorbent body comprising a water-absorbent resin and hydrophilic fiber, and the absorbent body is then thermally fused (see for example Patent Documents 6 and 7), a method in which the surface of a water-absorbent resin having anionic groups is coated with a cationic polymer, so that the particles adhere and are fixed to one another as the resin swells (see for example Patent Document 8 and 9), a method of using an emulsion binder to fix the water-absorbent resin and hydrophilic fiber, and a method of using a hot melt adhesive to fix the water-absorbent resin to a base material (see for example Patent Document 10 and 11) and the like. Because in such examples the particles of water-absorbent resin are stacked on each other on the support so as to increase the proportion of water-absorbent resin, the blocking effect is greater and a large quantity of binder must be used. When a large quantity of binder is used to fix the water-absorbent resin on the base material in this way, moreover, the binding force itself may restrict the swelling of the water-absorbent resin. In particular, the inherent absorbent abilities of the water-absorbent resin may not be fully exploited if the water-absorbent resin, hydrophilic fiber and the like are fixed with a thermoplastic binder or emulsion.
Techniques for reducing restrictions on the swelling of the water-absorbent resin when it is fixed on a support include a water absorbable composite material comprising a water-absorbent resin part of which is held within a bulky nonwoven fabric, while the exposed surface of the resin is coated with fine cellulose fiber and the outer surface of the composite is covered with a fibrillated hot melt (see for example Patent Document 12) and an absorbent composite sheet comprising a water-absorbent resin part of which is held within a bulky nonwoven fabric and part of which is exposed on the surface, wherein the outer surface of the resin is coated with a fiber net-like double hot melt layer consisting of two layers with different sized mesh (see for example Patent Document 13). However, although swelling is less restricted with these methods, the absorption properties are affected by the fixing process. As has already been shown, the absorption properties of water-absorbent resin include not only the absorption capacity, absorption speed, absorption capacity under pressure, dispersion-absorption capacity under pressure and liquid permeability of the swollen gel, but also the capillary absorption factor, which is an absorption property based on the capillary force in the gaps between particles (see for example Patent Document 14 and 15). It has been shown that this capillary absorption capacity and other absorption properties of water-absorbent resin are greatly affected by conventional fixing means. That is, even using highly functional water-absorbent resins the absorption properties of absorbent bodies obtained by fixing those resins often have not reflected the inherent absorption properties of the water-absorbent resins. It has also been said that it is not necessary to insist so much on the performance of the water-absorbent resin as long as it fulfills a minimum performance requirement (see for example Patent Document 16), and differences in the absorption properties of water-absorbent resin have in fact been hard to distinguish when the resin is made into an absorbent body, making it hard to differentiate absorbent bodies.
In some cases, an adhesive has been used to bond an absorbent gel to a chemically strengthened cellulose fiber (see for example Patent Document 17). In this method the chemically strengthened cellulose fiber ensures a space for swelling of the absorbent gel, effectively separating the water-absorbent resin particles and allowing the resin to function more easily. However, the fibers are not fixed to each other, and the fiber inevitably moves inside the absorbent body, resulting in a corresponding movement of the absorbent gel. Also, large quantities of cellulose need to be used because the absorbent gel must be enveloped in cellulose fiber to ensure space for swelling, and the swelling space itself is not adequate. A large quantity of adhesive must also be used, which inevitably restricts swelling. Adhesion has also been accomplished using a base material and a crosslinking agent (see for example Patent Document 18). In this method, it appears that an absorbent composite with good liquid permeability has been obtained by using a crosslinking agent which does not restrict swelling of the gel, along with a low weight of particles to prevent gel blocking. However, a crosslinking agent is required for adhesion, and the absorption capacity of the particles could be reduced when some of them are crosslinked. Moreover, it is also said that performance under load is improved when the degree of surface crosslinking is increased, but in this case the blocking prevention effect would not be adequate. In addition, the absorption capacity of the composite is low because a low weight of particles is used.
In some cases, a water-absorbent resin has also been fixed to a base material without using a binder. In one method, absorbent polymer particles are bonded to a fibrous base material during polymerization, and polymerization is performed on the fibrous base material (see for example Patent Document 19). In this method, the fibrous base material penetrates between the polymer particles and the particles are strongly fixed, but it is difficult to complete the reaction in the base material, and there is likely to be considerable residual monomer and residual crosslinking agent. In another example, a certain amount or more of an aqueous monomer solution is carried as fine particles on a raised nonwoven fabric, and then polymerized and thermally compressed (see for example Patent Document 20). The absorption performance of the composite is high in this case because of the large quantity of water-absorbent resin, and because a nonwoven fabric is used there is less movement than with pulp. However, because polymerization is performed in a nonwoven fabric there is still the problem of residual monomers. Another method is to re-impregnate a water-absorbent resin with an aqueous solution of unpolymerized monomer, apply it to a base material and polymerize the impregnated monomer to thereby bond the water-absorbent resin to the base material (see for example Patent Document 31), but it is difficult to completely polymerize the residual monomer after it adheres to the base material, and large quantities of residual monomers occur.
In a similar example, the water-absorbent resin is made into a slurry and applied to a base material (see for example Patent Document 21). Productivity is certainly improved by applying a slurry, but expensive microfibril fiber must be used as the dispersion medium, and the adhesive force may not be adequate. In thin sanitary materials and other absorbent materials with a higher proportion of water-absorbent resin, because more water-absorbent resin is used the resin can swell and become rather bulky after absorbing water depending on how it is positioned. The more strongly the water-absorbent resin is fixed, the more it presses on the body when it becomes bulky.
A method has also been proposed of printing drops of low-viscosity aqueous monomer solution on fabric, and then performing a polymerization reaction with the fabric to thereby provide gaps between polymerized particles and prevent gel blocking (see for example Patent Document 30). In this case, large amounts of residual monomers and low-molecular-weight components remain because of the difficulty of achieving complete polymerization on a base material. This not only makes the product undesirable for use as a sanitary material, but also reduces the absorption speed. When arranging resin on both sides of one sheet of fabric in an effort to achieve the desired level of absorption relative to area, moreover, the resin must have a large particle size of 550 μm or more, and the extremely small surface area/volume of the spherical, semi-spherical and deformed spherical particles obtained by this method does not provide sufficient absorption speed for actual use.
From the standpoint of comfort, it is important to prevent not only leakage but also dampness in a hygiene product. Absorbent products have been proposed in which dampness is reduced by controlling the rise in humidity that occurs during use (see for example Patent Document 22 and 23). In the techniques described in these publications, the absorbent bodies contain absolute dry pulp, a large quantity of highly-absorbent polymer and a hygroscopic material such as silica gel or lithium chloride, in combination with a moisture permeable back sheet. Some absorbent products also use a moisture permeable back sheet (see for example Patent Document 24). Another technique combines two moisture permeable sheets in order to prevent liquid from seeping through the moisture permeable back sheet even under pressure. However, because in these methods the secreted bodily fluid remains unfixed between the fibers of the paper or pulp, when a large amount of bodily fluid is excreted the unfixed liquid may produce steam, resulting in dampness.
There have also been proposed a sanitary napkin wherein rewetting from the absorbent body is prevented through the use of an absorbent body in which the centrifugal holding capacity after equilibrium absorption swelling of false blood and the false blood permeating speed are at or above a fixed value (see for example Patent Document 25), a multilayer absorbing paper having a surface layer to be first contacted with a liquid and made of mixed bulky cellulose fiber and one or more base material layers laminated to the surface layer (see for example Patent Document 26), and an absorbent sheet comprising hydrophilic fine fibers or a hydrophilic fine powder contained in an absorbent sheet comprising a highly-absorbent polymer and bulky cellulose fiber (see for example Patent Document 27). However, these publications do not describe the configuration of an absorbent article that allows steam to be dramatically controlled and humidity to be dramatically suppressed even when the amount of excretion (amount of liquid to be absorbed) is large.
Another method for controlling dampness uses fiber with a low water-holding capacity for the absorbent layer (see for example Patent Document 28). There is certainly less dampness with this method, but because the fiber functions hardly at all as an absorbent body, absorption is dependent entirely on an absorbent resin with a slow absorption speed, resulting in slower absorption. Because the fiber expands very little as it swells, moreover, gel blocking is likely and it is difficult for the water-absorbent resin to perform properly.
It has been reported that the absorbent capability of a water-absorbent resin is affected by the shape of the particles (see for example Patent Document 29). Commonly used water-absorbent resin particles are not especially long and thin and have a particle diameter of about 45 to 850 μm with a mass median particle diameter of about 200 to 370 μm, but the absorbent capability of a structure and the effective capability of the water-absorbent resin in it were improved by means of a relatively large particle size distribution, with a median particle diameter of 400 to 700 μm. Conventionally, however, when the absorbent capability is improved the effect of blocking cannot be avoided because there is more contact between particles. Because of the low proportion of water-absorbent resin, moreover, the absorbent capability of the absorbent body is low.
Thus, most absorbent bodies in which a water-absorbent resin is bonded to a base material use adhesion by monomer polymerization (raising the issue of residual monomers) or adhesion using an adhesive (which restricts swelling), and no satisfactory adhesion method currently exists. Moreover, only absorbent bodies with poor performance have been produced because the resin is not in a state that allows high absorption performance after adhesion. That is, no thin, lightweight absorbent body has been obtained having high liquid holding capability and absorption speed, high liquid dispersibility and high stability of the absorbent body.    [Patent Document 1] Japanese Patent Application Laid-open No. 2001-252307    [Patent Document 2] WO98/037149    [Patent Document 3] Japanese Patent Application Laid-open No. 06-057010    [Patent Document 4] WO01/64153    [Patent Document 5] Japanese Patent Application Laid-open No. 5-230747    [Patent Document 6] Japanese Patent Application Laid-open No. 10-118114    [Patent Document 7] Japanese Patent Application Laid-open No. 10-118115    [Patent Document 8] Japanese Patent Application Laid-open No. 5-31362    [Patent Document 9] Japanese Patent Application Laid-open No. 6-370    [Patent Document 10] Japanese Patent Application Laid-open No. 2000-238161    [Patent Document 11] Japanese Translation of PCT International Publication No. 10-510447    [Patent Document 12] Japanese Patent Application Laid-open No. 2001-96654    [Patent Document 13] Japanese Patent Application Laid-open No. 2001-171027    [Patent Document 14] Japanese Patent Application Laid-open No. 2004-001355    [Patent Document 15] Japanese Patent Application Laid-open No. PL365287    [Patent Document 16] Japanese Patent Application Laid-open No. 2001-96654    [Patent Document 17] Japanese Patent Application Laid-open No. 10-512183    [Patent Document 18] Japanese Translation of PCT International Publication No. 10-508528    [Patent Document 19] Japanese Patent Application Laid-open No. 2003-11118    [Patent Document 20] Japanese Patent Application Laid-open No. 2004-124303    [Patent Document 21] Japanese Patent Application Laid-open No. 11-137600    [Patent Document 22] Japanese Patent Application Laid-open No. 6-218007    [Patent Document 23] Japanese Patent Application Laid-open No. 7-132126    [Patent Document 24] Japanese Translation of PCT International Publication No. 10-508521    [Patent Document 25] Japanese Patent Application Laid-open No. 7-184956    [Patent Document 26] Japanese Patent Application Laid-open No. 6-287886    [Patent Document 27] Japanese Patent Application Laid-open No. 9-156013    [Patent Document 28] Japanese Patent Application Laid-open No. 2002-165837    [Patent Document 29] Japanese Patent Application No. 2904791    [Patent Document 30] US2003/0205318 A1    [Patent Document 31] Japanese Patent Application Laid-open No. 9-239912    [Patent Document 32] Japanese Patent Application Laid-open No. 2005-200630